Clear polymeric labels are increasingly desired, since they provide a no-label look to decorated glass and plastic containers. Paper labels block the visibility of the container and/or the contents in the container. Clear labels enhance the visual aesthetics of the container, and therefore the products, and are growing much faster than paper labels in the package decoration market as consumer product companies are continuously trying to upgrade the appearance of their products on store shelves.
A high degree of resistance to water whitening, especially hot water and/or very cold water, by pressure sensitive adhesives (PSAs) is required for some clear label applications. For example, bottles labeled with clear film PSA labels can be subjected to pasteurization conditions where the label is expected to remain clear throughout the process. In other labeling applications, bottles labeled with clear PSA labels are subjected to long-term submersion in very cold water, such as with beer and beverage bottles. Historically, solvent based adhesives have been used for such demanding applications. Environmental considerations, however, favor elimination of solvents. This and the higher coating speeds attainable with emulsion PSAs favor their use in place of solvent based PSAs.
Emulsion based PSA polymers are made in the presence of surfactants and other water soluble electrolytes such as initiators. In addition, comonomers such as unsaturated carboxylic acids are employed to impart or enhance adhesive properties and tend to be present at the latex-particle interface and remain there during film formation. Films cast from such emulsions are sensitive to water and become cloudy simply when a drop of water is placed on the film. Water whitening is almost always assured when the water is hot, e.g., the temperature of the water is 70° C. or higher.
The water resistance of acrylic emulsion polymer films has long been addressed in the art, particularly with respect to paint films. The art has observed that the presence of water soluble electrolytes, surfactants and polyelectrolytes such as sodium polymethacrylate cause or enhance water sensitivity. It is also known that surfactants are migratory species that with time move toward and bloom to the surface of films, also rendering them water sensitive.
The art has taught that the use of low levels of surfactant, a reduction or elimination in soluble electrolytes and crosslinking between the emulsion particles can all serve to improve or impart water resistance.